Lasers such as described in the U.S. Pat. No. 3,659,225 to Furumoto et al. use a high power flash to drive the laser. The near instantaneous, high voltage electrical input to such a flash lamp is provided by charging a large capacitor from line voltage to over fifteen kilovolts (kV). To fire the laser, the large capacitor is discharged into the flash lamp.
To provide the high voltage of over fifteen kV from a 110 V line voltage, a step up transformer is conventionally used. A transformer which could operate from line voltage and frequency would be unsuitably large. Thus, past charging supplies have first converted the ac line voltage to a dc voltage. The dc voltage is applied to a series inverter which generates an ac input to the charging transformer at a high frequency in the order of several kilohertz (kHz).
In a preferred series inverter charging circuit, two silicon controlled rectifiers (SCR) are connected across the dc power supply to drive the primary of the charging transformer in a push-pull configuration. To provide maximum efficiency the SCR gating frequency should be very close to but less than the resonant frequency of the charging circuit. In that way, reflective current returned through the primary circuit to the SCRs acts to positively turn one SCR off and also to re-enforce the current drawn through the transformer by the other SCR. Early SCR series inverter circuits used a fixed gating frequency throughout the charging period. This frequency could not be optimum throughout the charging because, in capacitor charging, the load impedance varies from zero to a very high value. With that change in impedance the resonant frequency changes continually, and at best any fixed frequency is a compromise.
Power supplies have been made more efficient by varying the switching frequency to match the load. With feedback responding to the charge level of the charged capacitor, the gating frequency of the SCRs can be made to closely match the resonant frequency of the charging circuit.
A more serious problem with series inverter switching supplies is that, if the gating frequency is not a predetermined amount less than the resonant frequency of the circuit, one SCR may be turned on before the other has stabilized in the off condition. The result is simultaneous conduction by the two SCRs and a short across the dc power supply. Because the switches are operated near the upper limits of their current handling capabilities, such a short circuit is likely to result in destruction of the SCRs. To minimize this problem, some series inverter circuits include circuits which sense this shorted condition and shut the inverter down. In an attempt to avoid such simultaneous conduction, the circuits are designed, based on the LC time constant of the circuit, to delay gating of each SCR for a predetermined time after the expected zero crossing of the ac charging current.